These solvents are used industrially in large quantities, in particular as reaction media for synthesis of organic compounds, as solvents for the manufacture and use of polymers, and in extraction of oil products. In most of these applications, the aprotic polar solvent ends up generally in the form of aqueous solutions which are often rich in dissolved inorganic salts (up to 10% by weight).
For reasons of economy and especially in order to protect the environment, it is particularly desirable to treat these aqueous solutions in order to recover the aprotic polar solvent in a sufficiently pure form in order to allow its reuse.
When it is desired to recover and to purify an aprotic polar solvent from its aqueous solutions, distillation is the most commonly used technique. However, this technique is not directly applicable to saline aqueous solutions of aprotic polar solvents with boiling points higher than that of water. In fact, as water, which is more volatile, is being evaporated, salts which are generally of low solubility in the aprotic polar solvent can generate dangerous distillation conditions by crystallising in an untimely manner in the boiler. Thus, when a saline aqueous solution is being dealt with, it is absolutely essential to carry out its desalting beforehand.
The method of desalting usually practised consists in processing the saline aqueous solution beforehand in a thin film evaporator where the aprotic polar solvent and water undergo flash evaporation and are then entrained, free from salts, towards the distillation column. This method has however the following drawbacks:
risks of decomposing the aprotic polar solvent on contact with the overheated walls of the evaporator, PA1 inevitable loss of solvent by entraining with the salts at the bottom of the apparatus, PA1 tricky operation because of the thickness of the layer of salt which must always be kept to a minimum in order not to damage the scraping mechanism (blades, motor), PA1 high investment cost and energy cost. PA1 it can be carried out at room temperature or at a slightly higher temperature (up to 60.degree. C.) and therefore does not risk spoiling the products, PA1 it allows recovery of more than 98% of the aprotic polar solvent introduced and complete separation of the salts, by providing, on the one hand an aqueous effluent with a high salt content and free from solvent and, on the other hand, an aqueous solution which is enriched in aprotic polar solvent and which can be processed by distillation or any other suitable separation technique, PA1 it is economical in terms both of investment and of operating costs, PA1 it can be used in continuous or batch mode, PA1 finally, it is safe to use.
Another technique which allows elimination of the salts before distillation is electrodialysis. Compared to the preceding technique, electrodialysis has the following advantages:
Application of electrodialysis to water-DMSO-NaNO.sub.3 mixtures has already been described in the Czechoslovakian Patent 169,049. In this case electrodialysis proceeds with no particular problem. This is no longer the case however when alkaline aqueous solutions are being dealt with which contain multivalent cations, such as Zn.sup.2+, made soluble in the form of complexes and capable of precipitating in an alkaline medium in the form of insoluble hydroxides. In this case basifying of the "brine" compartments of the ectrodialyzer and precipitation of zinc in this medium are seen. This precipitate blocks the small intermembrane space and then hinders the flow of the brine which is necessary for the electrodialyzer to function.
It has now been found that this problem can be solved if, during electrodialysis, the brine is kept at an acidic pH. Against all expectation, it has been noted that acidifying the brine in order to avoid precipitation of hydroxides of metals such as zinc is not accompanied by acidifying of the alkaline solution to be desalted and does not have a detrimental effect on the resistivity of the system or on the stability of DMSO (no decomposition into bad-smelling dimethyl sulphide).